(1) Field of the Invention
This invention relates to a fuel feed quantity control system for an internal combustion engine (hereinafter called "engine"), and particularly to a fuel feed quantity control system of the so-called L Jetronic system, which performs the control of fuel feed quantity by directly detecting the intake flow rate of the engine.
(2) Description of the Related Art
As conventional fuel feed quantity systems of such L Jetronic system, there are systems adapted to control the fuel feed quantity on the basis of intake flow rate information from an air-flow sensor of the Karman vortex detection type arranged in an intake passage of an engine.
The air-flow sensor of the Karman vortex detection type is arranged in the intake passage of the engine. As depicted in FIG. 40, the air-flow sensor 10 is composed of a flow straightener 1, a passage 2 with a sound absorbing material applied on the inner wall of the passage, a vortex generator (Brough body) 3, vortex-stabilizing plates 4, ultrasonic wave transmitter 5, ultrasonic wave receiver 6, and an electronic circuit (see FIG. 41).
Owing to the provision of the air-flow sensor, the inducted air is straightened by the flow straightener 1 and then flows through the passage 2, whereby a train of vortices is created by the vortex generator 3. The frequency of generation of these Karman vortices is proportional to the flow velocity of the inducted air. An ultrasonic wave of about 40 KHz, which has been transmitted from the ultrasonic wave transmitter 5 toward the ultrasonic wave receiver 6, is affected by the Doppler effect of the revolutionary flow of each Karman vortex, so that the propagation time from the ultrasonic wave transmitter 5 to the ultrasonic wave receiver 6 changes. Namely, this propagation time varies in accordance with a change in the frequency of generation of Karman vortices, which change in turn takes place due to a change in the flow velocity of the inducted air. Accordingly, the ultrasonic wave from the ultrasonic wave transmitter 5 is subjected to phase modulation by the flow velocity of the inducted air (namely, the flow rate of the inducted air) and is then received by the ultrasonic wave receiver 6.
In the electronic circuit, as illustrated in FIG. 41, an oscillator 7 delivers a pulse signal to the ultrasonic wave transmitter 5 so as to transmit the ultrasonic wave. At the same time, the oscillator 7 also outputs as a reference signal the same pulse signal as the aforementioned pulse signal to a phase comparator 9 via an average phase follow-up circuit (phase-shift circuit or subtractor) 8, and a signal outputted from the ultrasonic wave receiver 6 is delivered via an amplifier 12 to a phase comparator 9 where the output signal of the receiver is compared with the reference signal, whereby the output signal of the receiver is demodulated in phase and is thereafter outputted.
Since the above-described propagation time varies slightly depending on the temperature of the inducted air, a low frequency fraction (a fraction of frequencies lower than the frequency of generation of Karman vortices) caused due to variations of the intake air temperature is detected from signals outputted from the phase comparator 9 by means of a low-pass filter (loop filter) 11. By a detection signal thus generated, the average phase follow-up circuit 8 shifts the phase of the pulse signal from the ultrasonic transmitter 6 thereby to compensate the modulation by the intake air temperature.
Such control systems may be classified into the synchronous system, in which the state of operation of an engine is controlled in unison with the signal outputted from an air-flow sensor, and other systems.
Incidentally, the output frequency of the air-flow sensor (the frequency of generation of Karman vortices) f is expressed by the following equation: EQU f=Sr(u/d) (1)
where u: flow velocity, d: width of vortex generator and Sr Strouhal number. The Strouhal number Sr is in turn expressed as a function of the Reynolds number Re of a flow around the vortex generator 3 (FIG. 42). By the way, the Reynolds number Re is expressed by the following equation: EQU Re=u(d/.nu.) (2)
where .nu.: kinematic viscosity of air.
When the intake air temperature T.sub.a or intake air density (atmospheric pressure) P.sub.a varies, the kinematic viscosity .nu. of air changes as shown in FIG. 43 and accordingly, the Strouhal number St also changes so that the output frequency f of the air-flow sensor is altered. In FIG. 43, numbers shown in parentheses in FIG. 43 correspond to an atmospheric pressure of 75 mmHg while those indicated without parentheses correspond to an atmospheric pressure of 750 mmHg.
Further, the relationship between the output frequency f of the air-flow sensor and the flow rate (pulse constant) P.sub.c per pulse outputted from the air-flow sensor also varies with the intake air temperature or the atmospheric pressure. The manner of changes of the Pc-f characteristics for the intake temperature as a parameter may be illustrated as shown in FIG. 44.
It is appreciated that such a phenomenon becomes remarkable where the Reynolds number Re is small (i.e., smaller than 10.sup.3), as is apparent from FIG. 42 too.
Let's now consider the current correction of the air/fuel ratio. It is practised to correct the air density by the output of the air-flow sensor (the quantity of air inducted) on the basis of the intake air temperature or the air density which changes with the atmospheric air. However, no correction has been performed at all taking into consideration the fact that the output frequency of the air-flow sensor itself varies in accordance with changes in the kinematic viscosity .nu..
Accordingly, let's assume that the intake temperature has increased. Since the kinematic viscosity .nu. increases and the Reynolds number Re and Strouhal number Sr decreases, the output frequency of the air-flow sensor drops even when the flow velocity u is constant. This has led to a problem that the air/fuel ratio becomes leaner correspondingly.